Fifteen-year change in forest floor organic and element content and cycling at the Turkey Lakes Watershed

To assess the long-term effects of atmospheric deposition on forest floor c hemical composition, we took quantitative samplings of L-(Oi), F-(Oe), and H-(Oa) layers at an old-growth sugar maple-yellow birch stand on a till soi l at the Turkey Lakes Watershed near Lake Superior, Ontario, Canada, in 198 1 and 1996. We then assessed these samples for contents of organic matter ( OM), total N, K, Ca, Mg, S, and Na, and exchangeable NH4+, NO3-, K+, Ca2+, Mg2+, SO4,2- and Na+. Over the 15-year period, total OM and element content s remained unchanged, with the exception of N, which increased significantl y from 61.3 kmol/ha in 1981 to 78.4 kmol/ha in 1996. On an area basis, ther e were significant increases in exchangeable Ca2+ (from 3.8 to 4.6 kmol/ha) and Na+ (from 0.05 to 0.08 kmol/ha) and decreases in exchangeable NH4+-N ( from 1.41 to 0.95 kmol/ha) and SO42- -S (from 1.29 to 0.96 kmol/ha). There were no significant differences in average annual litterfall OM, N, Ca, Mg, S or Na inputs between 1980 and 1985 and between 1992 and 1997. Average an nual wet-only SO42- -S deposition during 1981-86 was 0.30; during 1992-97, it was 0.21 kmol/ha. Annual wet-only NO3- -N averaged 0.33 kmol/ha during 1 981-86 and was similar during 1992-97. Throughfall was less rich in SO42- a nd Ca2+, Mg2+, and Na+ during 1992-97 than earlier. Throughfall NH4+ and NO 3- fluxes were unchanged. Efflux of cations from the forest floor reflected reduced throughput of SO42-. Overall, the results suggest that in spite of atmospheric inputs, active biological processes-including litter input, fi ne-root turnover, and tree uptake-serve to impart stability to the mineral composition of mature sugar maple forest floor.